Impedance transformation means



N. E. LINDENBLAD 2,249,963

IMPEDANCE TRANSFORMATION MEANS Filed Nov. 15, 1939 July 22, 1941.

2 Sheets-Sheet 1 Z i" ii Z 22 INVEN TOR. N/LS E. LINDENBLAD BY M IATTORNEY.

' July 22, 1941. BL D 2,249,963

IMPEDANCE TRANSFbRMATION MEANS Filed Nov. 15. 1939 2 Sheets-Sheet 2 6 v22 z, .24 z, 26 1 L- -mp-J INVENTOR. N/LS E. L/NDENBLAD ATTORIYEY;

Patented July 22, 1941 IMPEDANCE TRANSFORMATION MEANS Nils E.LindenbladQ Port Jefferson, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application November 15, 1939, SerialNo. 304,573

16 Claims.

The present invention relates to transmission lines and, moreparticularly, to a method of, and means for, transforming the impedanceof one transmission line to such a value that it may be directlyconnected to another transmission line of a different impedance.

An object of the present invention is to provide a means for matchingthe impedance of one transmission line to that-of another.

Still another object of the'invention is to provide a means for matchingthe impedance of a pair of transmission lines without introducinglosses.

Still another object of the present invention is to provide a means fortransforming the'impedance of a transmission line without introducingcircuits of high volt ampere storage.

The present invention includes, among its features, a plurality of shortsections of concentric transmission lines which are designed to beconnected in series at one end and the series combination directlyconnected to one trans mission line. The other end of the sections areconnected in parallel and connected to the other transmission line. Insome instances, it may be necessary to interpose between certain of thesections and one of the transmission lines phase inverting circuits sothat the resultant phase of the currents in all of the sections willbe'the same at the junction point. By a structure constructed inaccordance with the present invention it is possible to obtainanimpedance multiplication which is the square of the number ofintermediate sections employed.

Other objects, features and advantages of the present invention willappear from the following detailed description, which is accompanied bydrawings in which Figures 1, 2 and 4 illustrate simplifiedtransformation systems useful in explaining the principles of thepresent invention; Figure 3 illustrates a simple embodiment of thepresent invention whereby an impedance ratio of 4 to 1 is obtained;Figure 5 illustrates another embodiment of the invention also resultingin a 4 to 1 impedance transformation ratio; Figure 6 illustrates anotherembodiment of the invention in which an impedance ratio of 9 to l isobtained; while Figure '7 illustrates a modification of the form of myinvenpedance of 2 to 1. The circuit shown in Figure 1 comprises aconcentric transmission lineA having an inner conductor I and an outershell 2. The inner and outer conductors of this transmission line areeach connected at one end to the center conductors 3 and 4 of a pair ofpushpull concentric line branches. An outer conducting sleeve 6surrounds the end of line A for a distance equal to a quarter of theoperating wavelength and is joined to the outer conductors 5 of thepush-pull branches as shown.

To understand clearly the operation of the circuit, let us visualize thecircuit from the lower end into which are fed currents of oppositeinstantaneous direction in the two push-pull branches 3 and 4, asindicated by the arrow marks. Since, in a single concentric conductorline the center conductor current and the shell current on the innersurface of the outer conductor are opposite to each other in directionbut of the same magnitude, it follows that the inner conductor 4 of onebranch of the push-' pull circuit should continue as the inner conductorI of the single concentric line A, while the inner conductor 3 of theother push-pull branch should continue as the shell 2 of the single lineA. To prevent short-circuitingof the push-pull branch connected to theouter conductor 2 of line A, it is necessary for the shell 2 of line Aat its end to present a high impedance on its outer surface to theoperating frequency. If this is so, all current arriving over conductor3 must travel over the inner surface of shell 2 of line A and none iscarried by shell 5 back to the outer conductor of line 3, The necessaryhigh impedance is obtained by making the length of shell 2 extendingwithin sleeve 6 a quarter of a wavelength long and connecting the upperend of sleeve 6 to the outer conductor of line A. Since lines 3 and 4are eifectively in series with respect to line A each must have a surgeimpedance equal to half the surge impedance of line A. An impedancetransformation of 2 to 1 is therefore obtained. a

Figure 2 shows a modification of the form of construction shown inFigure 1 wherein the outer shell 5 of the push-pull concentric line iscontinued through sleeve 6 as a single piece with a notch cut out of oneside in order to enablethe connection of the conductors of line Athereto. The operation of this figure is otherwise as described withreference to Figure 1. Figure 3 shows how Figure 2 may be modified inorder to connect the two balanced concentric lines 3 and 4 to a singletransmission line B for impedance shell 3|. 'shell'2 of transmissionline A. The current in the matching. A phase shifting loop I isconnected in series with one of the balanced lines, conductor 3, in thefigure, said loop I having a length equal to a half the length of theoperating wave. Therefore, at the point of connection of conductor 9 oftransmission line B the current in ly connected to branches 3 and 4 inparallel it.

must have an impedance equal to half that of each of the branches orZ/2. Thus, it will be seen we have an impedance transformation of 4 to1.

By involving another aspect of the present in-- vention it is possibleto connect a first transmission line to another line by bringing aconductor from the first line through the inside of the inner conductorand connecting it to the inside of the outer conductor of the other lineas well as a conductor from the outside of the outer conductor andconnecting it to the inner conductor. In order to preventshort-circuiting of the first line each of the conductor connectionsshould be a quarter of the length of the operating wave from the end ofthe other line where the inner conductor is bonded to the shell. I haveshown in Figure 4 how this principle of connection may be employed.Since the voltage required in the two branches containing conductors 3and 4 are of opposite phase, I use the U- shaped phase reversal circuitas described with reference to Figure 3. Each branch 3, 4 of the Ucircuit then contains a current I. The center conductor currents and thecorresponding shell currents in the two branches add up in the commonline A which they feed, as illustrated by the arrows. It will be notedthat the inner conductor 3 of one branch is connected to the inside ofthe outer shell 2 while the inner conductor 4 of the other branch isconnected to the outside of the inner conductor 1. The outer shell ofthe branch containing conductor 3 is, for convenience, in this figureshown as a continuation of the centercondu'ctor I of transmission lineA.

The voltage at the termination of thecommon line A is equal to that ofeach branch line 3 and 4. The impedance of the common line A thus has tobe half of the impedance of the branch lines. This figure simplyrepresents an exampleof one principle of my invention since theimpedance' of common line A is the same as that of common line B and isshown and described to 'make what follows more clearly understood. Thisexample shown in Figure 4 shows two ways of adding currents andtransmission lines at the same voltage. The branch lines 3 and 4 can beconsidered as added into transmission line A or as into transmissionline B.

In Figure 5 I show how the individual branch lines 3 and 4 may havetheir currents added in series. The current in the common line A is thesame as in the branches but the voltage is doubled. It should be notedthat branch line 3 enters the transmission line A as a quarter wave Theinner conductor 3 connects to the shell 5 of the left-hand branch thencontinues along the conductor passing through the aperture in wall 5|and becomes the center conductor current in conductor 4. The centerconductor current of 3 continues as a shell current of transmission lineA. The shell section 3| within the casing l and 2 is then anintermediary point at which the voltage appearing in conductor 3 isconnected. in series with that appearing in conductor 4. Due to thequarter wave length of shell 3! current is prevented from flowing alongits outside surface. Since the current in line A thus must be equal tothe current in each of the branches 3 and 4 and since the voltage is thesum of the voltages, the impedance of line A must be twice that ofeither of the branch lines.

It should also be noted that the currents in the branch lines are inphase requiring no phase reversal. They may be connected directly inparallel at their lower end and connected to conductors 8 and 3 oftransmission line B as shown. Thus, the total circuit from B to A ofFigure 5 represents an impedance transformation of 4 to 1 with a systemwhich is matched throughout.

In Figure 6 I have shown a system by means of'which a total impedancetransformation of 9 to 1 may be accomplished by using three branchesconnected in parallel to transmission line B and in series totransmission line A. As will be seen from an inspection of the drawings,each of the branches 22, 24 and 23 are surrounded by a shell 28 adistance equal to a quarter of the operating wavelength and connected atits lower end to the outer conductors of the branches. Theseintermediate shells, therefore, have a very high impedance on theiroutside to currents of the operating frequency. The current in thecenter conductor 23 of the first branch becomes the shell current forthe middle branch and the current of the center conductor 25 of thecenter branch becomes the shell current of the right-hand branch.Neither the current in conductor 23 which flows to the shell 24 of thecenter branch and the current in conductor 25 which flows to the shell26 of the right branch can go to the outside of the respective shell dueto the high impedance of the quarter wave section surrounding the shellbut must go on the inside and become the shell current as alreadystated. The shell current of the left-hand branch becomes the centerconductor current of the main line A while the center conductor currentof the right-hand branch becomes the shell current of the main line A.Since the three branches 22, 24 and 26 are connected in series and arein phase relationship, the impedance of each of these branches should beone-third of the impedance of the line A. At their lower ends each ofthe branches are connected in parallel and connected to the centerconductor 9 and shell 8 of transmission line B. The impedance oftransmission line B will thus have to be onethird of the impedance ofeach of the intermediate sections. The transformation ratio, therefore,becomes 9 to 1 between transmission line A and transmission line B.

In Figure 7 I have shown a modification of the form shown in Figure 6which is particularly significant in that the parallel and seriesbranches are so arranged that the transformation takes place as thewaves travel along the system. The construction of this modification mayin some cases be mechanically preferable to the previous modification.In this modification transmission line A is composed of an outer shell12 and an inner conductor H). The inner conductor I0 is carriedthroughout the length of the transformation section and is connected tothe inner conductor [9 of transmission line B. The lower portion ofconductor I0 is surrounded by a shell H which forms with conductor I!)one of the intermediate matching sections. Shell It is surrounded byanother shell l3 thus forming a second transformation section in whichshell H acts as the center conductor and shell I3 as the outerconductor. The upper end of shell II is connected to the upper end ofconductor 2 of transmission line 13. Due to the quarter wavelength ofshell II from its end to its point of connection with shell l3 thecurrent from conductor 20 must flow on the inner surface of shell ll.Likewise, in the upper portion of the figure shell l3 becomes the innerconductor of a transformation section in which the shell I2 of line Aforms the outer conductor. The end of conductor 2| is connected to theupper end of shell l3. The current from 2! is compelled to flow on theinner surface of shell 13 as described with reference to shell I! above.The impedance of line section 20 is so adjusted that the current at thejunction of 2D and I9 divides one-third to conductor I0 and two-thirdsto conductor 20. Likewise, the impedance of section 2| is so adjustedthat the current 2! at the junction of 20 and 2| divides equally.

Regardless of the path each portion of current takes as it is divided,at the points where the current-s are brought together they are again inphase, since the total path lengths are equal. I

The three portions of current added in series to feed transmission lineA result in a voltage of 3E and a total current of i as compared to thevoltage E and current 3i in transmission line B. A 9 to l impedancetransformation is therefore obtained.

The fundamental principle underlying all of these circuits is that acenter conductor current in one line may be made the shell current inanother line. If the current paths are so arranged that no gaps arepresented either to the center conductor current or the shell currentthe system operates to produce an impedance transformation without anyaccompanying voltampere storage.

Throughout this application, wherever I have spoken of lengths oftransmission line as being equal to a quarter of the length of theoperating wave. it should be distinctly understood that if it is desiredany odd multiple of that length may be used. A single quarter wavelengthhas been referred to since it is not only simple language but alsoresults in a more compact structure.

While I have particularly shown and described several modifications ofmy invention, it is to be prises coupling a plurality of short sectionsof transmission lines in a series relationship to one of said lines andin a parallel relationship to the other of said lines, the number ofsaid sections being equal to the square root of the transformation ratiorequired.

3. In a circuit including a pair of transmission lines of difierentimpedance, mean; for

transforming the impedance of one line to a value equal to that of theother comprising a plurality of short sections of transmission lines,said sections being connected in a series relationship to one of saidlines and in a parallel relationship to the other of said lines.

4. In a circuit including a pair of transmission lines of differentimpedance, the impedance of one line being N times that of the other,means for transforming the impedance of one line to a value equal tothat of the other comprising an integral number N of short sections oftransmission lines, said sections being connected in a seriesrelationship to said one transmission line and in a parallelrelationship to said other transmission line.

5. In combination, a first concentric transmission line having an innerconductor and an outer shell, said shell and conductor being connectedtogether at one end, a second and a third concentric transmission lineeach having one end extending into the said end of said firsttransmission line a distance equal to a quarter of the length of theoperating wave, the outer shell of said second line forming an extensionof the inner conductor of said first line, the inner conductor of saidthird line being connected to the shell of said first line, the innerconductor of said second line being connected to the shell of said thirdline whereby said second and third lines are efiectively connected inseries across the conductors of said first line.

6. In combination, a first concentric transmission line having an innerconductor and an outer shell, said shell and conductor being connectedtogether at one end, a second and a third con-centric transmission lineeach having one end extending into the said end of said firsttransmission line a distance equal to a quarter of the length of theoperating wave, the outer shell of said second line forming an extensionof the inner conductor of said first line, the inner conductor of saidthird line being connected to the shell of said first line, the innerconductor of said second line being connected to the shell of said thirdline whereby said second and third lines are efiectively connected inseries across the conductors of saidfirst line, means for connectingsaid second and third lines in parallel at their ends.

7. In combination, a first concentric transmission line having an innerconductor and an outer shell, said shell and conductor being connectedtogether at one end, a second and a third concentric transmission lineeach having one end extending into thesaid end 'of said firsttransmission line a distance equal to an odd multiple, including unity,of a quarter of the length of the operating wave, the outer shell ofsaid second line forming an extension of the inner conductor of saidfirst line, the inner conductor of said third line being connected tothe shell of said first line, the inner conductor of said second linebeing connected to the shell of said third line whereby said second andthird lines are effectively connected in series across the conductors ofsaid first line.

8. In combination, a first concentric transmission line having an innerconductor and an outer shell, said shell and conductor being connectedtogether at one end, a second and a third concentric transmission lineeachhaving one end extending into the said end of said firsttransmission line a distance equal to an odd multiple, including unity,of a quarter of the length of the operating Wave, the outer shell ofsaid second line forming an extension of the inner conductor of saidfirst line, the inner conductor of said third line being connected tothe shell of said first line, the inner conductor of said second linebeing connected to the shell of said third line whereby said second andthird lines are effectively connected in series across the conductors ofsaid first line, means for connecting said second and third lines inparallel at their other ends.

9; In combination, a first and a second transmission line each having aninner conductor and an outer shell, a plurality of short sections ofconcentric transmission lines connected in series at one end and to saidfirst transmission line, means surrounding said ends for preventingcurrent from flowing on the outside of the shells of said shortsections, said sections being connected in parallel to said second lineat their other ends.

10. In combination, a first and a second transmission line each havingan inner conductor and an outer shell, a plurality of short sections ofconcentric transmission lines connected in series to said. firsttransmission line, means surrounding said ends for preventing currentfrom flowing on the outside of the shells of said short sections, saidsections being connected in parallel to said second line. i

11. An impedance transformer comprising a high impedance section oftransmission line having an outer shell and an inner conductor, a secondshell surrounding said inner conductor extending within said outer shella distance equal to one quarter of the length of the operating wave andconnected thereto, a third shell extending within said second shell adistance equal to onequarter of the length of the operating wave andalso surrounding said inner conductor and connected to said second shellat the end thereof, a low impedance section of transmission line havingan outer shell and an inner conductor, means connecting the outer shellsof said first mentioned and last mentioned sections of transmission linetogether, and means for connecting the inner conductor of said lastsection of transmission line to the inner conductor of said firstsection of transmission line and to the inner ends of each of saidsecond and third shells.

12. An impedance transformer comprising a high impedance section oftransmission line having an outer shell and an inner conductor, a secondshell surrounding said inner conductor extending within said outer shella distance equal to one quarter of the length of the operating waveand-connected thereto, a third shell extending within said second shella distance equal to a quarter of the length of the operating wave andalso surrounding said inner conductor and connected to said second shellat the end thereof, a low impedance section of transmission line havingan outer-shell and an inner conductor, means connecting the outer shellsof said first mentioned and last mentioned sections of transmission linetogether, and means for connecting the inner conductor of said lastsection of transmission line to the inner conductor of said firstsection of transmission line and to the inner ends of each of saidsecond and third shells, said last mentioned means being so arrangedthat the current in said last mentioned line is divided equally.

13. An impedance transformer comprising a high impedance section oftransmission line having an outer shell and an inner conductor, a secondshell surrounding said inner conductor extending within said outer shella distance equal to a quarter of the length of the operating wave andconnected thereto, a third shell extending within said second shell adistance equal to a quarter of the length of the operating wave and alsosurrounding said inner conductor and connected to said second shell atthe end thereof, a low impedance section of transmission line having anouter shell and an inner conductor, means connecting the outer shells ofsaid first mentioned and last mentioned sections of transmission linetogether, and means for connecting the inner conductor of said lastsection of transmission line to the inner conductor of said firstsection of transmission line and to the inner ends of each of saidsecond and third shells, said last mentioned means being so arrangedthat the current in said last mentioned line is divided equally betweensaid first inner conductor and said second and third shells.

14. An impedance transformer comprising a high impedance section oftransmission line having an outer shell and an inner conductor, a secondshell surrounding said inner conductor extending within said outer shella distance equal to a quarter of the length of the operating wave andconnected thereto, a third shell extending within said second shell adistance equal to a quarter of the length of the operating wave and alsosurrounding said inner conductor and connected to said second shell atthe end thereof, a low impedance section of transmission line having anouter shell and an inner conductor, means connecting the outer shells ofsaid first mentioned and last mentioned sections of transmission linetogether, and means for connecting the inner conductor of said lastsection of transmission line to the inner conductor of said firstsection of transmission line and to the inner ends of each of saidsecond and third shells, said last mentioned means being so arrangedthat the current in said last mentioned line is divided equally betweensaid first inner conductor and said second and third shells, said lastmentioned means being so arranged that the lengths of current path fromsaid second line to said first line are equal.

15. An impedance transformer comprising a high impedance section oftransmission line having an outer shell and an inner conductor, a secondshell surrounding said inner conductor extending within said outer shella distance equal to an odd multiple, including unity, of a quarter ofthe length of the operating wave and connected thereto, a third shellextending within said second shell a distance equal to an odd multiple,including unity, of a quarter of the length of the operating wave andalso surrounding said inner conductor and connected to said second shellat the end thereof, a low impedance section of transmission line havingan outer shell and an inner conductor, means connecting the outer shellsof said first mentioned and last mentioned sections of transmission linetogether, and means for connecting the inner conductor of said lastsection of transmission line to the inner con-- ductor of said firstsection of transmission line and to the inner ends of each of saidsecond and third shells.

16. An impedance transformer comprising a high impedance section oftransmission line having an outer shell and an inner conductor, a secondshell surrounding said inner conductor extending within said outer shella distance equal to an odd multiple, including unity, of a quarter ofthe length of the operating wave and connected thereto, a third shellextending within said second shell a distance equal to an odd multiple,including unity, of a quarter of the length of the operating Wave andalso surrounding said inner conductor and connected to said second shellat the end thereof, a low impedance section of transmission line havingan outer shell and an inner conductor, means connecting the 10 outershells of said first mentioned and last mentioned sections oftransmission line together, and means for connecting the inner conductorof said last section of transmission line to the inner conductor of saidfirst section of transmission line and to the inner ends of each of saidsecond and third shells, said last mentioned means being so arrangedthat the current in said last mentioned line is divided equally betweensaid first inner conductor and said second and third shells.

NILS E. LINDENIBLAD.

